1. Field of the Invention
This invention relates to a device for automatically adjusting the sensitivity level of a sensor circuit for a power circuit control device. More particularly, the sensitivity level of a Doppler type approach or presence sensing system is varied inversely with the noise level of the environment.
2. Brief Description of the Prior Art
One fundamental approach to motion detection is found in U.S. Pat. No. 3,668,703, wherein a motion detector relying upon what is commonly known as Doppler shift detection, is disclosed.
The patent discloses that various intrusion detection systems are based on the concept of radiating energy such as ultrasonic or electromagnetic energy into the area to be detected and detecting the presence of an intruder by noting any change in the energy resulting therefrom. A highly satisfactory system based on the Doppler frequency effect had been developed. For example, U.S. Pat. No. 3,242,486 which is assigned to Johnson Service Co. discloses a triggered intrusion detection system employing the Doppler effect. Energy of a given frequency is radiated into the protected area and the reflected signals are received by a suitable receiver, along with a portion of the transmitter energy. Any movement of a body within the area results in a Doppler frequency output from the receiver unit. The Doppler frequency is detected and utilized to energize or trigger an alarm and thereby indicate the presence of an intruder.
The basic operation of a low-powered Doppler detector is based on the comparison of the transmitted and an echo signal. In the absence of an intruder or other target motion the transmitted and echo signals will have the same frequency. Any motion, however, results in a shifting in the frequency in the reflected or echo signal and not of the transmitted or original. The shift will be dependent upon the radial velocity of the intruder.
U.S. Pat. No. 2,393,014 to Bartholy and assigned to Westinghouse Electric Corporation and U.S. Pat. No. 3,459,961 to Ravas and assigned to Westinghouse Electric Corporation disclose that the foregoing Doppler shift principles can be applied to the control of power circuits, and in particular lights, through the use of a time delay system. These and similar patents relating to power circuit control devices do not distinguish between the requirements of a light control system and an alarm control system, but merely fall back on the available Doppler shift motion detection technology. It is recognized that there is a great similarity between the requirements for the two systems and one can incorporate an alarm circuit to trigger a mechanism subject only to a predetermined time delay. However, in commercial applications it has been found that the alarm systems requirements are not totally compatible with light control requirements, and accordingly in actual use difficulties have been encountered.